Natural gamma ray logging is one of the most common logging techniques in the oil and gas industry. Such logging operations measure the naturally occurring gamma radiation emitted by a subterranean formation, primarily from potassium, uranium, and thorium containing minerals in the formation. Both total gamma ray and spectral gamma ray sensors are commonly employed. Total gamma ray sensors measure the total number of detected gamma rays with an energy above a predetermined threshold. Spectral gamma ray sensors measure the energy spectrum of the emitted gamma rays and may be used to determine the weight concentrations of potassium, uranium, and thorium in the formation.
In operations in which the tool string further includes a neutron source, such as a pulsed neutron generator or a chemical neutron source as found in common nuclear logging tools, drilling fluids (mud) in the borehole may be activated by the generated neutrons. Such activated mud is known to emit gamma rays that may be detected by a natural gamma ray sensor.